1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is the bioactive hormone of the vitamin D endocrine system. It functions via the vitamin D receptor (VDR) to impact bone by increasing intestinal calcium absorption and by directly affecting osteoblast/osteoclast activity. The overall goal of this research proposal is to examine direct transcriptional effects of 1,25(OH)2D3 and VDR in osteoblasts. Toward this goal, expression arrays identified several novel genes that are induced by 1,25(OH)2D3 in osteoblastic cells. This proposal focuses on one 1,25(OH)2D3-induced gene that impact osteoblast biology, namely Meningioma-1 or MN1. MN1 is a transcription factor that serves as a coactivator in nuclear receptor- mediated transcription. Deletion of the MN1 gene in mice markedly impairs intramembranous ossification leading to defects in cranial skeletal development. These data demonstrate an important biological role for MN1 in skeletal homeostasis. Our preliminary data show that MN1 is expressed in numerous osteoblastic cell lines and its expression is regulated by 1,25(OH)2D3 in osteoblastic cells in vitro and in vivo. Ectopic expression of MN1 in osteoblasts inhibits osteoblast proliferation and shRNA- mediated knock-down of MN1 expression reduces osteoblast differentiation markers in vitro. These studies are the first to address MN1 function in skeletal homeostasis. A major hypothesis tested in this proposal is that VDR and 1,25(OH)2D3 are potent regulators of MN1 gene expression and of other genes that are required to maintain normal skeletal homeostasis. We propose to examine the significance of the vitamin D endocrine system in skeletal homeostasis by focusing on the continued characterization of MN1 in osteoblast function. Specifically, we propose to: 1) determine the significance of MN1 in development and maintenance of the appendicular and axial skeletons, 2) determine the molecular mechanisms involved in the transcriptional regulation of the MN1 gene by 1,25(OH)2D3 in osteoblastic cells, 3) establish the functional significance of MN1 in 1,25(OH)2D3-activated transcription in osteoblastic cells, and 4) determine the role of MN1 in controlling osteoblast proliferation and differentiation. These studies will provide new insight into the direct actions of the vitamin D endocrine system in skeletal homeostasis and an improved understanding of key factors that are needed to maintain skeletal integrity. PUBLIC HEALTH RELEVANCE: The molecular details involved in vitamin D effects in bone and osteoblasts are largely unknown. Our working hypothesis states that vitamin D controls the expression of the MN1 coactivator protein and impacts osteoblast function. The main goal of this proposal is to define novel target genes for VDR-RXR heterodimers in osteoblasts.